This invention relates to connections between rotary drives and rotary loads, wherein, due to parallel misalignment between the axes of the drive and load, such as, for example, orbital movement of the load axis during rotation, the drive and the load require a flexible connection. More particularly, the invention relates to a flexible-shaft-type connection between the drive and load that carries bending stresses during operation and especially to a means for isolating the end portions of the shaft at the point where the attachment is made to the rotary drive or rotary load from the stresses that occur due to the bending moment applied to the shaft.
One particular application where such flexible shafts are used is in connection with progressing cavity, positive displacement rotary pumps, such as the pumps described in my U.S. Pat. Nos. 3,512,904 and 3,938,744 (hereinafter referred to as "Allen" devices, or pumps). These pumps have a rotor with an exterior helical surface that engages the surrounding interior helical surface of the stator, the rotor surface having one more thread than the stator surface and a lead twice that of the stator surface. Thus, the stator surface and the rotor surface define therebetween sealed pumping cavities that are axially advanced as the rotor rotates and at the same time orbits in the same direction at two or more times the rate of its rotation. For a more complete description of Allen pumps of this type, reference is made to my aforesaid U.S. Pat. No. 3,512,904.
Another class of rotary helical devices or pumps that utilize flexible coupling shafts to advantage is the class that includes the well-known Moineau-type device as disclosed, for example, in U.S. Pat. No. 1,892,217. Typical examples of the use of flexible shafts for Moineau-type pumps are shown in U.S. Pat. Nos. 2,028,407; 2,456,227; and 3,612,734. The devices shown use metal shafts or metal cables.
More recently, flexible shafts formed of an engineering grade plastic, such as an acetal homopolymer known commercially as "DELRIN", have been used as disclosed in my U.S. Pat. No. 3,938,744. The connection of the flexible shaft to the rotary drive shaft and to the rotor can be accomplished in several ways, one method being, where space permits, radial flanges or hubs. Since it is usually not practical to form the shaft and hubs as an integral unit, it is often desirable to secure the hub or hubs to the shaft by bonding, such as, for example, in the case of engineering grade plastics, by spin welding.
During operation, however, the flexing of the shaft and the sharp change in flexibility at the joint between the shaft and built-in support collar or hub produce concentrated stresses in the shaft at the edge of the joint. This stress concentration in the bonding zone increases the danger of failure during use and shortens the effective life of the shaft.
The device of the present invention, however, reduces the difficulties indicated above and affords other features and advantages heretofore not obtainable.